An electric power steering system which assists in steering by driving a motor to decrease the load on the driver is such as to include an input shaft linked with a steering member (a steering wheel, a hand-operated steering wheel), an output shaft linked with steered road wheels via a pinion and a rack or the like, and a connecting shaft which connects the input shaft with the output shaft, whereby a value of steering torque that is exerted on the input shaft is detected by a torque sensor through an angle or torsion generated in the connecting shaft, so as to drive and control the steering assist motor which is linked with the output shaft based on the steering torque value so detected.
In recent years, in electric power steering systems like this, brushless DC motors have come to be used. In a brushless DC motor, since a current caused to through a stator is PWM (Pulse Width Modulation) controlled so as to generate a rotating magnetic field according to a rotational position of a rotor, a detector is incorporated for detecting a rotor position.
Patent Document No. 1 discloses means for detecting a short circuit failure of a motor and means for breaking a connection when a short circuit failure is detected.
Patent Document No. 2 discloses means for detecting a short circuit and means for cutting power supply to a driving circuit by a fail-safe relay upon detection of the short circuit.
Patent Document No. 3 discloses means for detecting a short circuit failure mode of a motor driving system.
Patent Document No. 4 discloses method for detecting an abnormal operation in a motor driving system.    Patent Document No. 1: JP-A-2004-106664    Patent Document No. 2: JP-U-6-42533    Patent Document No. 3: JP-A-6-233450    Patent Document No. 4: JP-A-10-167086